Nonmicrophonic tube



y 1934- K. c. DE WALT ,960,050

NONMICROPHONIC TUBE Filed Sept. 26, 1930 fig.

In ventor Kenneth C. De Walt,

MWze/k,

H is Attorney.

Til

Patented May 22, 1934 UNITE srAss ATENT OFFICE NONMICROPHONIC TUBE Kenneth C, De Walt, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York The present invention relates to electrodes, more particularly to grid structures for electron discharge devices of the small receiver type.

In devices of the type mentioned which com- 5 prise an evacuated envelope containing an incandescent cathode, an anode and a grid electrode interposed between the cathode and anode, it has been customary to form the grid by winding a plurality of closely adjacent turns of fine wires about a'pair of metal uprights. The lower ends of these uprights are sealed in the stem of the envelope. The filament is usually supported independently of the grid from a glass rod or bar which is held rigidly in place by suitable support rods. This arrangement of filament and grid members together with their sup-- ports has been found to be very practical from the manufacture and economy standpoints, particularly when made on a quantity production basis, but in operation there is a strong tendency for microphom'cs to be set up within the tube. Microphonic noise is always undesirable but more particularly so when the tube is used as a high gain amplifier in connection with photophone recording or other sensitive form of receiver. Moreover, microphonic action is accentuated in those cases where the tube is subject to vibration as in train control apparatus. It is highly desirable therefore to eliminate all sources or" miorophonics within the tube.

I have discovered that in the type of construction referred to, a considerable part of the microphonic action centers about the control member. It appears that the uprights which are relatively long and made of small gage wire for economy and electrical reasons tend to spring together or otherwise move when the tube is subjected to external vibration during operation. The movement of these members allows the convolutions of the helix to bow outwardly in a vibratory manner and change their spacial relation with respect to one another, thereby causing the characteristics of the tube to vary. An imperceptible vibration or other change in the position of the uprights may produce a radical change in the performance of the tube.

I propose, in accordance with my invention, to prevent the relative movement of the grid uprights by means of simple, inexpensive but nevertheless an efiicacious expedient which lends itself to quantity production methods. This object is attained by welding or otherwise aflixing a heavy metal brace rod across the upper end of the grid supports and preferably across both ends thereof. For a more complete understanding of the invention reference should now be made to the following specification and the accompanying drawing in which Fig. 1 represents a view in perspective and partly broken away of a tube improved in accordance with my invention and Fig. 2 is a plan view, also in perspective, looking down on the top of the tube shown in Fig. 1. As illustrated, the tube has a tipless envelope 1 which terminates in a reentrant stem 2. The filament 3 may be M-shaped and supported at the bottom at three places by a plurality of wires 4 sealed in the stem. The upper apexes of the filament are supported by a pair of resilient hooks 5 which are secured to rigid wires 6, the latter being fused into a glass rod or bar 7, shown in Fig. 2. This rod constitutes a common support and spacing member for ,the upper end of the electrodes and is held rigidly in position by means of two uprights 8. The latter are secured to a screw clamp 9 which embraces the stem; the clamp also carries a pair of rods 10 which support an anode 11. The anode is of the usual closed periphery type provided with flanged extensions which are bent around rods 10. In addition to the filament referred to, the anode encloses the grid electrode, the improved construction of which constitutes the subject of the present invention. Conductors are taken as usual from the various electrodes to contact pins 12 secured in the base 13. The grid or control member is constituted of very fine wire preferably of nickel, in order to obtain the proper desired operating characteristics and is Wound as a helix about two uprights 14 which may also be of nickel and which are sealed in the stem. The uprights are notched to receive the turns of the helix so as accurately to maintain the proper spacing of the convolutions.

When a device of the construction described hereinbeiore is employed in a high gain amplifying set, particularly in places where it is subjected to excessive vibration, microphonic noises are generated within the tube. 1 have discovered that a considerable part of these microphonics have their origin in the grid which is found to vibrate over a wide range of frequency. Under these conditions, the grid uprights tend to vibrate and allow the convolutions of the helix to bow outwardly, thereby changing the space relation between the grid wires, also between the grid and the remaining electrodes, The relative movement within the grid structure causes a variation in the electrical characteristics of the tube which in turn give rise to the undesirable noises referred to. In accordance with the present invention, I provide a heavy metal brace 15 across the upper ends of the grid uprights to which it may be secured in any suitable manner, e. g. by welding. The brace member is constituted preferably of the same material as that of the uprights, e. g. nickel, and is bent in an arcuate fashion, as shown more clearly in Fig. 2, so as to clear the filament supports. A similar brace may also be welded across the lower portions of the grid structure, further to prevent relative movement of the uprights, although in practice I have found that the upper brace alone is generally suificient for this purpose. Frequency spectrum tests have shown that whereas prior to providing a grid of the type specified with one or more improved braces, a tube exhibited grid bowing, as evidenced by the presence of microphonics in steps over a range of 75 to 2000 cycles, after the tube had been provided with braces, there was substantially no grid bowing, and hence no micro- .phonics, at frequencies higher than 200 cycles.

Resonant vibrations below 200 cycles are caused by vibration of the electrode assembly as a whole and cannot be eliminated entirely by an improved grid structure. The brace is easy to apply and may be readily denuded of gas during manufacture, furthermore, is relatively inexpensive.

While I have described my invention as pertaining more especially to grid electrodes, it will be understood that the transversely extending brace may be secured between any two uprights which tend to move as a result of external vibrations. For example, a brace may be secured to the upper end or both ends of the anode rods 10 in the event that the material of the anode 11 is so thin as to fail to ofier suflicient lateral support.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electron discharge device comprising an envelope which terminates in a stem, said envelope containing a plurality of cooperating electrodes including a grid, means for supporting the electrodes from the stem, said grid being constituted of a wire helix wound on a pair of uprights, an insulating member mounted at a. position offset from the grid and stem, supporting connections between said member and the electrodes, and a rigid brace connected between the grid uprights at their ends opposite the stem.

.2. An electron discharge device comprising an envelope which terminates ina stem, said envelope containing a plurality of cooperating electrodes including a grid, means for supporting the electrodes from the stem, said grid being constituted of a wire helix wound on a'pair of uprights, an insulating member mounted at a position oiTset from the grid and stem, supporting connections between said member and the electrodes, and a'rigid metal rod connected between the grid uprights at their ends opposite the stem.

KENNETH C. DE WALT. 

